1. Technical Field of the Invention
The present invention relates generally to a power supply control system for a heater used to heat a gas sensor such as a gas concentration sensor up to a desired activation temperature which may be employed in an air-fuel ratio control system for automotive vehicles for measuring the concentration of gas such as O2, NOx, or CO.
2. Background Art
Limiting current type gas concentration sensors are known which are used for measuring NOx contained in exhaust gasses of automotive engine. There is used one of such gas concentration sensors which includes a pump cell and a sensor cell. The pump cell works to pump oxygen (O2) contained in gasses admitted into a gas chamber out of the sensor or to pump oxygen (O2) of outside gasses into the gas chamber. The sensor cell works to measure the concentration of NOx contained in the gasses after passing through the pump cell. The pump cell and the sensor cell are designed to produce current signals indicative of the concentration of oxygen and NOx upon application of voltage thereto.
The above type of gas concentration sensors usually have disposed therein a heater for keeping the pump cell and the sensor cell at a desired activation temperature. The power supply to the heater is regulated under feedback control by monitoring the resistance value of a solid electrolyte body (which will also be referred to as a sensor element resistance value below) on which the pump cell and the sensor cell are disposed and determining the power supply so as to bring the monitored resistance value into agreement with a target one corresponding to the desired activation temperature.
An undesirable change in characteristic of the gas concentration sensor due to, for example, deterioration of the solid electrolyte body will, however, result in a shift between an actually controlled temperature of the pump and sensor cells (which will also be referred to as a sensor element temperature below) and the desired activation temperature. This may cause the pump and sensor cells to be overheated or less heated, so that they are kept out of the desired activation temperature, which will result in an undesirable variation in amount of oxygen pumped by the pump cell, thus leading to a great error in determining the concentrations of oxygen and NOx.
Specifically, when the sensor element temperature is increased above the desired activation temperature, it will cause the pump cell to dissociate NOx as well as oxygen within the gas chamber, which will cause the concentration of NOx to be determined as being lower than an actual value. Alternatively, when the sensor element temperature is decreased below the desired activation temperature, it will cause the pump cell to dissociate oxygen incompletely, which will result in an increase in amount of oxygen remaining within the gas chamber. The amount of oxygen dissociated by the sensor cell together with NOx is, thus, increased, thereby causing the concentration of NOx to be determined as being higher than an actual value.
Usually, the pump cell produces a current of several mA indicative of the concentration of oxygen, while the sensor cell produces a current of several xcexcA indicative of the concentration of NOx. A shift in amount of oxygen pumped by the pump cell from a target one, therefore, result in a great error in determining the concentration of NOx using the current produced by the sensor cell.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a heater control system for a gas concentration measuring apparatus which is designed to keep a gas concentration sensor at a desired activation temperature, thereby ensuring required measurement accuracy of the gas concentration measuring apparatus.
According to one aspect of the invention, there is provided a heater control apparatus for a gas concentration measuring system which may be employed with an automotive control system designed to control the quantity of fuel injected into an internal combustion gasoline engine as a function of an output of the gas concentration measuring system under feedback (F/B) control to bring the air-fuel (A/F) ratio into agreement with a target value. The gas concentration measuring system has a gas concentration sensor which consists essentially of a gas chamber, a first cell working to pump thereinto oxygen molecules contained in gasses admitted into the gas chamber and discharge the pumped oxygen molecules, a second cell working to determine a concentration of a specified oxygen containing gas component contained in the gasses having passed through the first cell, a solid electrolyte member on which at least one of the first and second cells is disposed, and a heater heating the first and the second cells. The heater control apparatus comprising: (a) a sensor element resistance value determining circuit working to applying one of a varying voltage and a varying current to the solid electrolyte member and measuring resulting changes in voltage appearing across the solid electrolyte member and current flowing through the solid electrolyte member to determine a resistance value of a sensor element including the solid electrolyte member; (b) a heater control circuit controlling a power supply to the heater as a function of a difference between the resistance value determined by the sensor element resistance value determining circuit and a target resistance value; (c) a heater power determining circuit determining a power supplied to the heater; and (d) a correcting circuit working to determine a reference resistance value of the sensor element as a function of the power determined by the heater power determining circuit based on a predetermined fundamental relation between a power used in the heater and a resistance value of the sensor element, the correcting circuit correcting the target resistance value as a function of the reference resistance value.
In the preferred mode of the invention, the correcting circuit corrects the target resistance value based on the reference resistance value and the resistance value determined by the sensor element resistance value determining circuit. Specifically, the correcting circuit may correct the target resistance value as a function of a difference between the reference resistance value and the resistance value determined by the sensor element impedance determining circuit.
The correcting circuit may reflect an external factor of a change in temperature of the sensor element in an environmental condition of the gas concentration sensor in correcting the target resistance value.
The correcting circuits may correct the target resistance value under condition that an external factor of a change in temperature of the sensor element is unchanged.
The gas concentration sensor may be installed in an exhaust pipe of an engine to measure and used in an engine control system working to detect exhaust gasses of the engine through the gas concentration sensor. The correcting circuit may correct the target resistance value under condition that the engine is at rest.
The degree to which the correcting circuit corrects the target resistance value may be stored in a backup memory as a learning value for use in correcting the target resistance value.
A measure of deterioration of the gas concentration sensor may be determined based on the degree to which the correcting circuit corrects the target resistance value.
According to the second aspect of the invention, there is provided a heater control apparatus for a gas concentration measuring system. The gas concentration system has a gas concentration sensor which consists essentially of a gas chamber, a first cell working to pump thereinto oxygen molecules contained in gasses admitted into the gas chamber and discharge the pumped oxygen molecules, a second cell working to determine a concentration of a specified oxygen containing gas component contained in the gasses having passed through the first cell, a solid electrolyte member on which at least one of the first and second cells is disposed, and a heater heating the first and the second cells. The heater control apparatus comprises: (a) a sensor element resistance value determining circuit working to applying one of a varying voltage and a varying current to the solid electrolyte member and measuring resulting changes in voltage appearing across the solid electrolyte member and current flowing through the solid electrolyte member to determine a resistance value of a sensor element including the solid electrolyte member; (b) a heater control circuit controlling a power supply to the heater as a function of a difference between the resistance value determined by the sensor element resistance value determining circuit and a target resistance value; (c) a heater resistance determining circuit determining a resistance value of the heater; and (d) a correcting circuit working to determine a reference resistance value of the sensor element as a function of the resistance value determined by the heater resistance determining circuit based on a predetermined fundamental relation between a resistance value of the heater and a resistance value of the sensor element, the correcting circuit correcting the target resistance value as a function of the reference resistance value.
In the preferred mode of the invention, the correcting circuit may correct the target resistance value as a function of a difference between the reference resistance value and the resistance value determined by the sensor element resistance value determining circuit.
The correcting circuit may reflect an external factor of a change in temperature of the sensor element in an environmental condition of the gas concentration sensor in correcting the target resistance value.
The correcting circuits may correct the target resistance value under condition that an external factor of a change in temperature of the sensor element is unchanged.
The gas concentration sensor may be installed in an exhaust pipe of an engine to measure and used in an engine control system working to detect exhaust gasses of the engine through the gas concentration sensor. The correcting circuit may correct the target resistance value under condition that the engine is at rest.
The degree to which the correcting circuit corrects the target resistance value may be stored in a backup memory as a learning value for use in correcting the target resistance value.
A measure of deterioration of the gas concentration sensor may be determined based on the degree to which the correcting circuit corrects the target resistance value.
According to the third aspect of the invention, there is provided a heater control apparatus for a gas concentration measuring system. The gas concentration system has a gas concentration sensor which consists essentially of a gas chamber, a first cell working to pump thereinto oxygen molecules contained in gasses admitted into the gas chamber and discharge the pumped oxygen molecules, a second cell working to determine a concentration of a specified oxygen containing gas component contained in the gasses having passed through the first cell, a solid electrolyte member on which at least one of the first and second cells is disposed, and a heater heating the first and the second cells, The heater control apparatus comprises: (a) a sensor element resistance value determining circuit working to applying one of a varying voltage and a varying current to the solid electrolyte member and measuring resulting changes in voltage appearing across the solid electrolyte member and current flowing through the solid electrolyte member to determine a resistance value of a sensor element including the solid electrolyte member; (b) a heater control circuit controlling a power supply to the heater as a function of a difference between the resistance value determined by the sensor element resistance value determining circuit and a target resistance value; (c) a sensor element temperature determining circuit determining a temperature of the sensor element; and (d) a correcting circuit working to determine a reference resistance value of the sensor element as a function of the temperature determined by the sensor element temperature determining circuit based on a predetermined fundamental relation between a temperature of the sensor element and a resistance value of the sensor element, the correcting circuit correcting the target resistance value as a function of the reference resistance value.
In the preferred mode of the invention, the gas concentration sensor may be installed in an exhaust pipe of an engine to measure and used in an engine control system designed to detect exhaust gasses of the engine. The sensor element temperature determining circuit may determine, as the temperature of the sensor element; one of an actual temperature of the sensor element and an estimate thereof based on one of a temperature and a flow rate of the exhaust gasses.
The control circuit controls the power supply to the heater to bring a temperature of the sensor element into agreement with a target controlled temperature. The sensor element temperature determining circuit determines two temperatures of the sensor element defined across the target controlled temperature. The correcting circuit determines two reference resistance values corresponding to the two temperatures of the sensor element by look-up using the predetermined fundamental relation and corrects the target resistance value using the two temperatures of the sensor element and the two reference resistance values.
The correcting circuit corrects the target resistance value as a function of a difference between the reference resistance value and the resistance value determined by the sensor element resistance value determining circuit.
The correcting circuit may reflect an external factor of a change in temperature of the sensor element in an environmental condition of the gas concentration sensor in correcting the target resistance value.
The correcting circuits may correct the target resistance value under condition that an external factor of a change in temperature of the sensor element is unchanged.
In a case where the gas concentration sensor is installed in the exhaust pipe of the engine and used in the engine control system, the correcting circuit may correct the target resistance value under condition that the engine is at rest.
The degree to which the correcting circuit corrects the target resistance value may be stored in a backup memory as a learning value for use in correcting the target resistance value.
A measure of deterioration of the gas concentration sensor may be determined based on the degree to which the correcting circuit corrects the target resistance value.